   [dtb]
   (let [files (get-files dtb)
         bytes (->> files (map #(.length %)) (reduce +))]
-    (-> bytes (/ 1024) Math/round)))
+    (-> bytes (/ 1024) math/round)))
 
 (defn depth
   "Return the depth of a given DAISY Talking Book.

  (:require [clojure.java.io :refer [file]]
            [clojure.string :as s]
            [clojure.tools.logging :as log]
            [pantomime.mime :refer [mime-type-of]]
            [mdr2.production.path :as path]
            [clojure.xml :as xml]

     (map file-audio-length)
     (reduce +)
     (* 1000)
     math/round)))

(defn- file-audio-channels
  "Return the number of audio channels for a given audio `file`"
